This invention relates to a scroll type fluid displacement apparatus, and more particularly, to an improved anti-wear device for the scrolls used in a scroll type fluid compressor.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 issued to Creux discloses the basic construction of a scroll type fluid displacement apparatus including two scrolls each having a circular end plate and a spiroidal or involute spiral element. The scrolls are maintained at an angular and radial offset so that both spiral elements interfit to form a plurality of line contacts between their curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scrolls shifts the line contacts along the spiral curved surfaces and, as a result, the volume of the fluid pockets increases or decreases, dependent on the direction of the orbital motion. Thus, a scroll type fluid displacement apparatus may be used to compress, expand or pump fluids.
In comparison with conventional compressors of the piston type, scroll type compressors have certain advantages, such as fewer parts and continuous compression of fluid. However, one of the problems with scroll type compressors is ineffective sealing of the fluid pockets. Axial and radial sealing of the fluid pockets must be maintained in a scroll type compressor in order to achieve efficient operation. The fluid pockets are defined by line contacts between the interfitting spiral elements and the axial contacts between the axial end surface of one spiral element and the inner end surface of the facing end plate.
Various techniques have been used in the prior art to solve the axial sealing problem. In U.S. Pat. No. 3,994,635, a seal element is mounted on the axial end surface of each spiral element and urged toward the end surface of the facing end plate by an urging device to effectuate sufficient axial sealing between the axial end surface of the spiral element and the end surface of the facing end plate. But, in the above '635 patent, because the seal element is urged toward the end surface of the facing end plate by an urging device, over a period of time, abrasions occur between the seal elements and the end plate of the scroll, especially when lightweight alloys such as aluminum alloys are used as the scroll material. When the end plate wears due to abrasion, the seal elements also are damaged, and the axial contact between the end surface of the spiral element and the inner end surface of the end plate becomes imperfect, which reduces compressor efficiency.
One solution to the above disadvantages is disclosed in commonly assigned copending patent application Ser. No. 587,871, filed Mar. 14, 1984. The '871 patent application discloses an anti-wear device for the scrolls which includes an anti-wear plate disposed on an end surface of the end plate of at least one of the scrolls. The anti-wear plate faces the axial end surface of the spiral element of the other scroll to prevent wear and maintain axial sealing.
As shown in FIG. 1, which is a vertical cross-sectional view of an earlier version of a scroll type fluid compressor, anti-wear plates 41' are disposed on an axial end surface of each end plate 271', 281'. Shims 113' are provided to establish a predetermined axial clearance between the axial end surface of each spiral element and the opposing anti-wear plate. Shims 113' are disposed between front end plate 11' and cup-shaped casing 12'. Even though the shim thickness is properly selected in the initial state, the axial clearance between the axial end surface of each seal element and the opposing anti-wear plate may change due to bending of each end plate in response to pressure changes. Also, once the thrust race and thrust balls of the rotation prevention/thrust bearing device settle due to a continuous compression gas load, the tight seal between the axial end surface of the seal element and the opposing anti-wear plate may be lost. Additionally, the spiral element, in particular, the central portion of the spiral element, expands in response to thermal changes in the compressed fluid causing further loss of sealing.
Though solutions exist for loss of axial sealing due to bending of the end plate and settling in the ball coupling type rotation prevention/thrust bearing device, no acceptable solution exists for changes in the axial length of the spiral element due to thermal changes. Thus, despite the existence of solutions to the other problems mentioned above, effective axial sealing between the seal element and the anti-wear plate can be easily lost.
Furthermore, it should be noted that, in scroll type fluid compressors, the interfitting spiral elements extend through several temperature zones. These different temperature zones are created because there are a plurality of pairs of sealed off fluid pockets between the interfitting spial elements, each of which has a different temperature and pressure. Because temperature and pressure at the central fluid pocket are the greatest, the central portion of each spiral element usually is the highest temperature and pressure area. Therefore, in order to achieve effective axial sealing in the central portion of the spiral elements, changes in axial clearance due to thermal changes must be minimized. However, to solve this thermal problem in the central portion of the spiral elements, if the initial axial clearance between the seal element and the anti-wear plate is set to a minimum, the central portions of the spiral elements strongly contact the opposing anti-wear plate resulting in abnormal wearing of the spiral elements and excessive force acting on the base portion of each spiral element.